The present invention relates to a fuel cell system having at least one or more fuel cells, wherein the fuel cell extends between a first cell end and a second cell end in tubular form, wherein the fuel cell is mechanically received with the first cell end on an inflow distributor unit, and wherein a fuel gas flows through the fuel cell, entering the first cell end and exiting one of the cell ends as exhaust gas.
Fuel cell systems of the type of interest here concern so-called SOFCs (Solid Oxide Fuel Cells). Fuel cells of this type have a ceramic electrolyte body of tubular form, the tube either being open at both ends or having a closed end side. The tubular basic form can be formed, for example, by electrolyte-supported fuel cells (ESC—Electrolyte-Supported Cells) or by anode-supported fuel cells (ASC—Anode-Supported Cells). The tubular basic form is therefore prescribed either by the electrolyte body or by an anode body. By virtue of the tubular form with two open ends or one closed and one open end, these can therefore be distinguished from planar designs of fuel cells.
The core of such fuel cell systems is formed by cells, in which a fuel gas (e.g. methane, hydrogen, carbon monoxide or a mixture) reacts, with the evolution of current and heat, to form carbon dioxide and water. In this case, natural gas which has been entirely or partially converted beforehand, depending on the system concept, by catalytic pre-reforming to form hydrogen has to be fed to the anode side. In this case, air is fed to the cathode, the cathode usually being applied on the outside in the case of tubular fuel cells, and the fuel gas can flow through the tubular fuel cell and electrochemically interact with the anode applied to the inside of the fuel cell.
Tubular fuel cells are mechanically received on a distributor unit, generally referred to as a manifold. If the fuel cells have an open form, the fuel gas is usually introduced through the first cell end, via which the fuel cell is mechanically received on the distributor unit, and exits the fuel cell via the opposing, second cell end. The fuel gas therefore flows from one end to the other end of the tubular fuel cells, it being possible for a plurality of fuel cells to be received on a distributor unit. If the fuel cells are closed at the second, free cell end, a lance is introduced into the tubular body of the fuel cell, such that the fuel gas, according to the prior art, is guided through the lance to the closed end of the fuel cell, in order to then flow back between the lance and the inside of the electrolyte body past the anode again in the direction toward the distributor unit.
Tubular fuel cells generally have a greater service life since the single-sided clamping and the small sealing length between the fuel cell and the distributor unit avoid thermomechanical stresses. Primarily, seals which are generally formed from solder glass can have a greater stability in the case of operating times of 40 000 to 150 000 hours if they do not have to be arranged in the hot region of the fuel cell system. To increase the service life, it would be expedient, however, to dispense with the use of seals entirely.
In order to obtain uniform feeding of fuel gas into the entire fuel cell and sufficiently effective distribution of the fuel gas in the cell, it is necessary to feed more fuel gas than can be utilized electrochemically via the anode wall for generating current. The ratio of converted fuel gas to fed fuel gas is referred to as the gas utilization level. Excess fuel gas can be burned in an afterburner. It is thereby no longer available for the current generation. Alternatively, excess fuel gas can in some cases be mixed with the fresh fuel gas and fed back to the stack, which is referred to as recirculation. This achieves the advantage that a higher gas flow prevails in the fuel cell system, without it being necessary to burn additional fuel gas. This enables operation with a greater service life combined with a high degree of efficiency. In this case, it is generally the case that between 50% and 90% of the exhaust gas is recirculated.
Fuel cells which are open at the ends give rise to the problem of feeding some of the exhaust gas which contains unburnt fuel gas back to the distributor unit for recirculation. As a result of thermal expansion and further thermal effects, it is often disadvantageous to mechanically clamp the tubular fuel cells both via the first cell end and via the second cell end, for example in order to receive and to dissipate the exhaust gas exiting the second cell end. Consequently, all of the exhaust gas is often burned with a high proportion of fuel gas which has not been utilized by electrochemical reaction.
The object of the invention is therefore that of providing a fuel cell system having improved fuel gas circulation. In particular, it is the object of the invention to provide a fuel cell system having an increased service life. In addition, it is the particular object of the invention to feed unburnt fuel gas for recirculation in an advantageous manner, where at the same time mechanical and thermal loading of the fuel cell and of appropriate sealing elements is minimized.